The present invention relates to the general field of turbine engines fitted with one or two unducted fans, and more particularly it relates to controlling the pitch of the fan blades of such engines.
A preferred field of application for the invention lies in open-rotor turbojets having two contrarotating propellers, which may be located relative to the gas generator, either downstream in a “pusher” configuration, or upstream in a “puller” configuration. Nevertheless, the invention also applies to turboprops having one or more propulsive propellers.
In a turbojet with contrarotating propellers, it is known that the pitch of the blades constituting the propellers constitutes one of the parameters enabling the thrust of the turbojet to be controlled, in particular by ensuring that the propeller always operates under the best possible conditions. Specifically, the speed of rotation of the propellers is practically constant during all stages operation, and it is the pitch of the propeller blades that varies thrust. Such pitch variation serves in particular to enable the propeller to operate under the best possible conditions. Thus, during a stage of cruising flight, it is desired to obtain the lowest possible power on the turbine shaft that is needed in order to obtain given traction at a given speed of the airplane so as to obtain best efficiency (i.e. the efficiency that serves to minimize fuel consumption and increase range). Conversely, during takeoff, the strongest possible traction is sought in order to accelerate the airplane and then cause it to take off.
The mechanism for controlling the pitch of the propeller blades of the turbojet is generally incorporated inside the hub carrying the propellers. More precisely, the pitch of each blade making up a propeller is typically controlled by a radial control shaft that passes through a casing arm in coincidence with the pitch axis of the blade and that has a lever arm at its inner end for enabling it to be turned about the pitch axis. An actuator that is stationary relative to the structures of the engine and that is centered on the longitudinal axis of the engine, then serves to drive movement in translation of the inner ring of a load transfer bearing (LTB) that is positioned in line with the actuator. The bearing serves to transmit the movement in translation from the stationary reference frame associated with the actuator to a rotary reference frame associated with the blades. Furthermore, for each control shaft, a pitch control rod connects the end of each lever arm to the outer ring of the LTB. Reference may be made in particular to Document WO2013/050704, which describes an implementation of such control.
That type of mechanism requires access to the hubs of the propellers of the engine in order to assemble the various elements making it up. However, for certain turbojet architectures, in particular for turbojets with contrarotating propellers in a pusher configuration, such access is limited by the fact that the hubs are masked by the rotary casings that reconstitute the air-flow passage through the engine. Specifically, with such a turbojet architecture, the radial control shafts need to be engaged via the inside of the hub carrying the propeller through casing arms and then to be coupled by means of fluting to the blade roots. In order to achieve this, it is therefore necessary for the space available inside the hub carrying the propeller to have a diameter that is at least equivalent to the length of the control shafts.
However, since it is desired to reduce the diameters of turbojet propeller hubs in order to improve the propulsive efficiency of propellers, it is becoming more and more difficult to insert the control shafts via the insides of the hubs.